Sound reproducing device



July 5, 1932. DE FOREST SOUND REPRODUCING DEVICE Filed May 22 1926 2 Sheets-Sheet 1 IN VEN TOR f 5 5% 0 A .J v Y B July 5, 1932. L DE FOREST SOUND REPRODUCING DEVICE Filed May 22 1926 2 Sheets-Sheet 2 INVENTOR (De/5 161 15] A) ATTORNE Patented July 5, 1932 i I 1,866,090

j, N D STATES PATENT OFFICE ':Appl1catlon fled Kay 22, 1926. Serial No. 110,880.

, This invention'relates broadly to sound reangle less than 180 and driven by a pluproducing devicessuch asphonographs and rality of motive means. I radio loud speakers. I I Fig. 3 is a cross sectional view of a modi- One of the objects of this invention is the fication of my invention, taken on the line provision of a sound reproducing device com- 33 of Fig. 10. I 55. prising'a plurality of membranes. 3 Fig. 4 is a further modification taken on .A further object of this invention is the the line 4- 1 of Fig. 12. construction of a device of this nature having Fig. 5 is a still further modification of my 'a plurality of curved membranes of the same invention, taken on the line 55 of Fig. 13. radii but made, of materials of difi'erent F 1g. 6 ShOWS a still further modification 0f thickness. t my invention.

A further object of the invention is the Fig. 7 is a different embodiment of the construction of a sound reproducingdevice principles of my invention. having a plurality of membranes of difierent Fig. 8 discloses a still further modificanatural periods of vibration. 7 tion, in which each membrane is driven by A still further object of this invention is its own motive means. the production of aloud speaker having more Fig. 9 is a perspective view of one form of than one vibrating membrane, which are conmotor adapted to drive the plurality of memstructed with different radii. branes. 0v Another object of thisinvention is the pro- Fig. 10 is a perspective view of one form 20 duction of aloud speaker having a plurality of mounting for the sound reproducing cleof curved membranes arranged so as to provice of this invention.

du a hor ff t, A v f Figs. 11, 12 and 13 represent other forms A still further object of thisinvention is in which the device may be embodied. the production of a loud speaker,.con sisting It has been found in actual practice, that l of a plurality of membranes adapted to be in the production of a sound reproducing de driven by one or more motive means. vice for use in connection with phonographs, Astill further object of this invention is radio loud speakers, and public address 1 the construction of a sound reproducing despeakers, considerable trouble is experienced vice having a plurality of membranes arin producing devices of this type which will ranged so as toproduce no load on the stylus. cover the desired frequency range. In the It. is another object of this invention to type of sound reproducing apparatus now produce a speaker unit with a'plurality of in common use, it has been practically immembranes adapted to be arranged to propossible to embody in a single instrument, a vide a given amount of surface in a given vibrating membrane which is adapted to i amountof space. i t I cover therange of frequencies ordinarily en- I Theseand many other objects result from countered in systems employing them. the inventiondisclosedherein,as will appear While developing my sound reproducing more clearly in the following description. device as disclosed in my Patent No. 1,560,502,

This invention residessubstantially in the of November 3, 1925, I discovered that it is i construction, arrangement, combination, and possible to construct a loud speaker adapted relative location of parts, as will beclearly to cover a wide range of frequencies, by em- 7 describedbelow. ploying a plurality of membranes driven by Referring to the drawings; one or by a plurality of vibration producing Fig.1 illustrates one form of my invention, units. I wish it to be understood at this which comprises. a plurality of membranes point, that in referring to loud speakers, I having the same radii, and driven by a single intend to include all types of sound reproducmotive means.. i ing devices such as may be used in connection r Fig. 2 is a view of another modification with phonographs, radio horns or speakers,

illustrating. two membranes subtendingan, and public address systems, which include branes of different radii, the same results can be secured. The invention is not confined to any specific membrane materials. How-' ever, such materials as paper or suitably treated cloth are suggested.

Similar advantages result from using membranes of different curvatures, as well as by using membranes of the same curvature, but driving them from arms of different radii.

' cured tangentially to the adjacent abutting" In Fig. 1, there is shown one embodiment of my invention, in which I employ the two curved membranes 1 and 2, which subtend an angle of 180. The outer edges ofthese membranes are secured in supports 3 and 4, and the inner adjacent edges are secured together by any suitable means, as gluing, etc. An electro-magnetic motor 5 is shown for influencing a membrane, that is, which is seedges of the membrane by means of a stylus 7 which includes a keeper 6 for thepoles' of the motor 5. As I disclosed in my above mentioned patent, I have found that a sound re' production of very clearquality is secured when the vibrations are imparted to the mem branes tangentially.

The arrangement in Fig. 2 employs two curved membranes 1 and 2, secured rigidly together at their inner edges, in a rigid support 8. These membranes subtend an angle less than 180 and greater than 90. Rigidly secured to the outer edges of these mem branes, are the arms 13, pivoted at 14, and secured by means of the link or stylus 12 to the armatures 11 of the electro-magnetic motor means 9. The construction of these motors is not described herein, since they form no part of this invention, but are of the usual differential type well known in the art. 1 In the arrangement shown in this figure,

these motors are electrically connected together in series, by means of the wires 15. Here, as in the arrangement shown in Fig. 1, the vibrations are imparted tangentially to the members. Where the membranes are made of the same material, and have the same fiexure, their rates of vibration may be made to differ by varying the point of application of the stylus 12 to the pivoted arm 14. Thus by this arrangement, one membrane can be adapted to handle the lower frequencies of vibration, and the other membrane the higher frequencies of vibration.

In the arrangement shown in Fig. 3, the two membranes 1 and 2 are curved to subtend an angle of 90, and are rigidly secured at their outer edges in the frames 3 and 4. I have indicated-generally at M, a motor secured to the back of the framework '16, which supports the members 3 and 4. The motor is connected to the two membranes by means of the styluses 17, in any suitable manner.

I wish it to be understood at this point that while I have shown, for the purposes of illustration,'two membranes, I intend to include under the term a plurality of membranes, two or more, which in some cases might run up as high as ten membranes where a very high quality of reproduction is required.

Inthe arrang'ement of Fig. 4, the two membranes 1 and 2 are curved to subtend an angle of 180, and are connected at their inner adjacent edges by means of theyoke 17 and motor M. In this arrangement, the space between theadjacent edges of the membranes is desirable since it prevents the locking of the air within the supporting casing 16, which might tend to produce undesirable effects on the quality of the sound reproduced.

I have shown at Fig. 5, an embodiment of my invention employing membranes 1 and 2 of different radii'whichmaybe made of the same or different material. The motor M is connected bymeans of the stylus 17 and yoke 18, through the link 19 to the inner adjacent" edges of the membranes 1 and 2, by any suitable arrangement, such as riveting, gluing, etc.

The two curved membranes 1 and 2, of the device ofFig. '6, are secured at their outer edges to the rigid supports 3 a-nd'4, and are connected at their inner adjacent edges by means of'the two styluses 26, to the arm 25, which is resiliently secured to a fixed support 24. The armature 20 is shown diagrammatically, since it forms no part of this invention. The'armature is connected by means of the'stylus 22 to the arm 25. It will be noted that the arm 25, being pivoted to the styluses 26 at one end, vibrates about this point, and since the styluses 26 are located at different distances from the point of sup- .port, theywill vibrate at different amplitudes, so that one membrane will vibrate at a lower rate of speedfthan the other. By

means ofthis arrangement, a wide range of frequency may be readily covered. It will be readily apparent that more than two membranes can be used in such an arrangement, a as well as with the other arrangements described above, where this is desired.

7 In .the embodiments disclosed, I have discoveredthat a still further valuable result is secured by means of aplurality of membranes,namely a horn effect; that is, the sound reproduced by the various membranes arranged as disclosed, is directed away from the membranes. In the forms shown in Figs. 6 and 8, Where a space is left between the adjacent edges, the surrounding air is permitted free passage between the membranes and does not result in the formation of undesirable sounds. However, where the mem-v branes are secured rigidly together, as in Figs. 1 and 2, I have found that they may be readily secured to a vibration transmitting element 28, which may be a bar of wood or other suitable material which is secured to the motor M by means of the stylus 27. In order to prevent the formation of undesirable sounds due to the formation of a closed air pocket, a damper element 29 is disposed between the membranes, as shown in Fig. 7. This membrane may be made of felt, cotton, or other suitable material.

In Fig. 8, is shown. a modification similar to that of Fig. 2, in which a plurality of electro-magnetic motors 30, having their armatures 31 mounted on the reed element 32 and connected by means of the styluses 33 to the edges of the membranes 1 and 2. This figure shows more clearly, the differential membranes are arranged so that their tendtype of motor referred to as being wellknown in the art.

Where it is desired to use a plurality of membranes of the same curvature and constructed of the same material, a motor of the type shown in Fig. 9 may be employed, which comprises the casing 40, having mounted therein electro-magnets 41. Secured across the top of the casing 40, and adjacent the ends of the electro-magnets, is a strip of magnetic material 42, having a slot 43 extending longitudinally thereof and to one side of the center line. Styluses 44 are secured to the strip 42 on opposite sides of the slot. It will be evident that by placing the slot to one side of the center line, the portions of the strip to which the styluses are secured, are of different width and therefore of different natural periods of vibration. By applying this motor to the embodiments of my invention of the type shown in Figs. 3 to 8 inclusive, it is possible, Where membranes of the same radii and construction are used, to impart to them different rates of vibration.

It will be evident from the drawings, that I intend operating a loud speaker, comprising a plurality of membranes, by means of a single or a plurality of motive means.

I am, of course, aware that many changes in the details of construction and arrangement of the elements of my invention will readily occur to those skilled in the art, and I do not, therefore, desire to be limited to the particular formations disclosed by way of illustration. For instance, the membranes may be made in many different forms, sizes, shapes, curvatures, etc., and of many different materials, without departing from the spirit of my. invention. I therefore wish to be limited only as required by my appended claims. 1

What 1 seek to secure by United States Letters Patent is:

1. A sound reproducing device comprising a plurality of curved membranes having different radii of curvature, and a sound reproducing member tangentially attached to at least one of the marginal edges of each of the membranes, and a frame for supporting said membranes and said member.

2. A sound reproducing device comprising in combination a plurality of curved membranes of different natural periods of vibration and means for reproducing sound attached to said membranes to impart tangential vibrations thereto.

3. A sound reproducing device comprising in combination a plurality of membranes of different natural periods of vibration and a sound reproducing member attached to the marginal edges thereof.

4. A sound reproducing device comprising in combination a plurality of curved membranes of different natural periods of vibration arranged with adjacent edges and means for imparting different rates of vibrations to the adjacent edges thereof.

5. A sound reproducing device comprising in combination a plurality of curved membranes of different natural periods of vibration and a plurality of means for imparting sound vibrations thereto.

In testimony whereof I have hereunto set my hand on this 18th day of May, A. D., 1926.

, LEE DE FOREST.

The illustrations in Figs. 10 to 13 inclusive,

are merely shown as methods ofmounting my sound reproducing device, and are given in the illustrative sense, to show the many forms to which it may be adapted.

It will also be evident that in the constructions of the types shown herein, no stress is imposed upon the stylus or styluses, since the Y ency to flatten out is opposed. By this arrangement, therefore, no external load is applied to the motor means to affect its operation. 

